Cleanrooms are essential to high-technology sectors including semiconductor fabrication, precision machine engineering, photographic industry, the production of pharmaceuticals and biological cleanrooms in medical institutions such as hospitals and the use of cleanrooms has expanded to the food industry and agricultural fields, as well as to their peripheral areas. While humidity, moisture and air streams are important environmental conditions in those industrial sectors, air purification is no less important. To purify air used in those sectors, HEPA (high-efficiency particulate air) filters composed of glass fibers and more efficient ULPA (ultra-low penetration air) filters are used. Neutral fibers, coarse dust filters, etc. that are composed of synthetic fibers other than glass fibers are extensively used as pre-filters to those high-efficiency filters. The filters mentioned above are mainly intended to remove particles, so they are designed to be capable of efficient removal of fine particles about 0.1 .mu.m in size. However, they are incapable of removing gases and ions.
The contamination of wafer surfaces in operating LSI fabrication plates is believed to occur due not only to fine particles but also to gases and ions. Contamination by gases and ions causes serious problems by increasing the contact resistance or affecting the bulk characteristics of wafers. Contaminating gases and ions can originate in various ways, such as in fabrication steps such as etching, from materials finished in cleanrooms and during the introduction of ambient atmosphere. Although the air in cleanrooms is constantly circulated, gases and ions once generated are not removed by the air purification system, so it is suspected that accumulating gases and ions may affect not only the quality of the final products but also the health of operating personnel.
Separation functional fibers or ion-exchange fibers produced therefrom are capable of effective adsorptive separation of heavy metal ions such as cobalt, nickel, mercury and copper ions contained in the process water used in precision electronics industry, medical field, pharmaceutical industry, nuclear power generation and food industry, as well as in the waste water discharged from these fields (see Japanese Patent Public Disclosure No. Hei 5-111665). According to Japanese Patent Publication No. Hei 5-67325, Japanese Patent Public Disclosure NOS. Hei 5-111607 and Hei 6-142439, filters made of ion-exchange fibers are capable of removing not only fine particles but also H.sub.2 S, NH.sub.3, carbon dioxide and hydrogen fluoride in gases. However, these prior fibers do not provide an adequately efficient removal of such substances from the gases.
A process for the production of ion-exchange fibers is described in Japanese Patent Publication No. 6-20554, corresponding to U.S. Ser. No. 08/264,762, which gives examples of adsorbing hydrogen chloride and ammonia in air atmosphere by means of the ion-exchange fibers. It would be desirable, however, to provide a more efficient adsorption of hydrogen chloride and ammonia,
The rate of adsorption and ion-exchange increases as the surface area and the density of functional groups in the surface increase. This is because adsorption and ion-exchange reactions always start at surfaces of the fibers and move gradually to the interior. In other words, one cannot say that the functional groups within the fibers are fully utilized. Hence, it is advantageous for the purposes of adsorption and ion-exchange that functional groups be densely present on surfaces of the fibers.
Due to the ease in controlling the site of the graft polymerization, radiation-initiated graft polymerization is attractive as a process for the production of functional materials and ion-exchange fibers and adsorbents produced by this method are under review. Radiation-initiated graft polymerization is commonly performed by a pre-irradiation liquid-phase process in which a substrate is first exposed to an ionizing radiation and then immersed in a monomer solution for reaction. In the early stage of the reaction, graft polymerization occurs in molecules at the surface of the substrate and its nearby area and progresses into the interior as the reaction time passes. However, it is difficult to insure that graft polymerization takes places in the surface of the substrate and its nearby area at an adequately high graft ratio in excess of 100%. This is also true with vapor-phase graft polymerization, which progresses into the substrate at high graft ratio. Hence, it has been difficult, even by radiation-initiated graft polymerization, to produce fibrous functional materials that have functional groups such as ion-exchange groups to be concentrated on the surface.
Another problem with the progress of reaction into the substrate in radiation-initiated graft polymerization is that if the substrate is formed of polypropylene, its physical strength decreases and it may undergo oxidative deterioration to release decomposition products. The most pertinent known prior art in this regard is a process for producing a gas adsorbent that has ion-exchange groups introduced into a polypropylene fiber substrate without a core-sheath structure by radiation-initiated graft polymerization (see Japanese Patent Publication No. Hei 6-20554).
Generally speaking, the larger the surface area of materials such as ion-exchangers and adsorbers that have separating capability, the higher the rate of exchange or adsorption and, hence, the more advantageous. Hence, the frequency of using ion-exchangers and adsorbers in the form of fibers having large surface areas is increasing. To take ion-exchange fibers as an example of fibrous materials having separating capability, there are known fibers of a multi-core structure comprising a polystyrene matrix or sea and polyethylene multi-filament cores or islands within the matrix, in which ion-exchange groups have been introduced into the matrix, and polyvinyl alcohol fibers having ion-exchange groups introduced therein after firing.
Example 3 of Japanese Patent Public Disclosure No. Hei 5-64726 shows that excellent results were attained when composite fibers comprising a polypropylene core and a polyethylene sheath, grafted by styrene monomer and then converted, were used as ion exchange fibers in an electrically regenerating desalinator; therefore, it is clear that ion-exchange fibers of a core-sheath structure exhibit high performance even in an electrically regenerating desalinator. However, if fibers having the polypropylene core are used in air, the polypropylene core will experience a drop in strength while undergoing decomposition, but researchers have shown that stability in air atmosphere can be attained by replacing the polypropylene with polyethylene terephthalate having high resistance to radiations and oxidation.
Most of the gas adsorbing filters known in the art are made from activated charcoal or zeolite treated with chemicals or supports carrying manganese oxides. Recently, gas adsorbing filters made from ion-exchange fibers have been developed for use in cleanrooms. The filter proposed in Japanese Patent Application No. Hei 4-294501 uses ion- exchange fibers of high-polymer resins that have been produced by radiation-initiated graft polymerization and it has proved to be very effective when used in cleanrooms.
Japanese Patent Application No. Hei 4-294501, supra, teaches a method of purifying micro-contaminated air in cleanrooms using nonwoven fabric filters made of high- polymer ion-exchange fibers produced by radiation-initiated graft polymerization. That patent gives example of using a nonwoven fabric of polypropylene fibers and a nonwoven fabric of a composite of polyethylene and polypropylene as the substrate to be treated by ionizing radiations; however, both polypropylene and polyethylene are prone to generate acidic substances under irradiation and in the presence of oxygen and, furthermore, they are susceptible to deterioration and dusting under irradiation.
In the years to come, cleanrooms will certainly be required to meet more strict standards on the cleanliness of air than permissible today, so the generation of dust and gaseous substances from the filter per se will obviously be a greater problem than it is today. To deal with this situation, the present invention adopts a core/sheath structure in which the core is made of a high-polymer component that is less prone to generate radicals and/or undergo degradation upon exposure to ionizing radiations, with the sheath being formed of a high-polymer component that is apt to generate radicals upon irradiation.